Physics · Lesson 18

Cathode Ray Tube

Electromagnetism Thermionic Emission Electron Beam Deflection

Heat a metal filament in a vacuum, and electrons boil off. Accelerate them with an electric field, focus them into a beam, and steer them with electric or magnetic fields. This is the cathode ray tube — the technology behind every oscilloscope, radar display, and television screen before flat panels.

Who Was J.J. Thomson?

J.J. Thomson (1856–1940) discovered the electron in 1897 using a cathode ray tube, measuring the electron's charge-to-mass ratio (e/m) by balancing electric and magnetic deflection forces. He showed that cathode rays were streams of particles far smaller than atoms — the first subatomic particle ever identified. He was awarded the Nobel Prize in Physics in 1906. His discovery launched atomic physics and fundamentally changed our picture of matter.

Core Concepts

Thermionic Emission & Acceleration

A heated cathode emits electrons. An anode at higher potential V accelerates them. All the electrical potential energy converts to kinetic energy:

½mv² = eV → v = √(2eV/m)

e = electron charge = 1.6×10⁻¹⁹ C

m = electron mass = 9.1×10⁻³¹ kg

V = accelerating voltage (volts)

Electric Deflection

Deflection plates create a transverse electric field E. The beam is deflected by force F = eE as it passes between the plates.

y = eEL² / (4mV)

E = electric field between plates (V/m)

L = length of deflection plates (m)

y = vertical deflection at end of plates

Magnetic Deflection

F = evB

B = magnetic flux density (Tesla)

v = electron speed (m/s)

Magnetic force is always perpendicular to velocity → circular arc

Interactive Simulator

Adjust voltage and deflection in the panel. Switch between Static Dot and Oscilloscope modes.

Beam Voltage5 kV

Deflection V0 V

Screen PosCentre

Real-World Applications

📡

Oscilloscopes A CRT oscilloscope deflects the beam vertically by the signal voltage and sweeps it horizontally at a set rate to display waveforms.

🎯

Radar Displays Early radar systems used rotating CRT beams to paint returning echoes on a phosphor screen, showing aircraft positions.

📺

Cathode Ray Televisions The beam scanned each line left-to-right, top-to-bottom 25–30 times per second, painting a full picture on the phosphor screen.

🔬

Electron Microscopes A focused electron beam at nanometre scale reveals atomic-level structures impossible to see with visible light.

Practice Problems

Use e = 1.6×10⁻¹⁹ C, m = 9.1×10⁻³¹ kg.

Easy1. Electrons in a CRT are emitted from the cathode. True or False?

Hint: The cathode is the heated negative electrode that emits electrons by thermionic emission.

Easy2. What happens to an electron beam passing through an electric field perpendicular to its motion?

Hint: A perpendicular force changes direction, not speed — the beam curves (deflects).

Medium3. An electron is accelerated through 1000 V. What is its speed in m/s? Use ½mv²=eV. Give answer to 2 significant figures (×10⁷).

Hint: v = √(2×1.6e-19×1000 / 9.1e-31) ≈ 1.87×10⁷ m/s. Enter 1.87.

Medium4. In magnetic deflection, F = evB. If B=0.01 T, v=2×10⁷ m/s, e=1.6×10⁻¹⁹ C, what is the force in ×10⁻¹⁴ N?

Hint: F = 1.6e-19 × 2e7 × 0.01 = 3.2×10⁻¹⁴ N. Enter 3.2.

Challenge5. Thomson balanced E=10⁴ V/m and B=0.01 T so the beam went straight (eE = evB). What is the electron speed in ×10⁶ m/s?

Hint: eE = evB → v = E/B = 10⁴/0.01 = 10⁶ m/s. Enter 1.

Innovator Profile · See Also

☢️ Marie Curie — Radioactivity, X-Rays & Atomic Decay